Topic: Potting Compost

[Tee Gee]

I think most of us on the forum are aware that suppliers of potting composts will not be allowed to add 'Peat' to the mix. At least this is the case for the compost sold to the general public, it is still unclear if this ruling will apply to commercial growers.

As has been reported over the past few years on various gardening related forums, some of the potting composts being sold as 'peat reduced ‘or 'peat free' have left a lot to be desired. Add to that, this year (2023) has seen massive rises in the cost of purchasing such composts.

For example, when these products first appeared on the market, they retailed at around 6p-7p per litre now it is around 10p-11p a litre a rise of around 50%-60% for which in many cases is still for rubbish!

To this end, particularly since giving up my allotments I have started to use containers to grow quite a lot of my stuff, and I am finding that it is becoming quite an expensive hobby!

To try and subsidise these costs, I have been experimenting for the last couple of years making my own potting composts!

The idea came to me when I looked back on my 50+ gardening years and found that I used the same soil every year and basically all I did to it was added humus (farmyard manure) and fertiliser to rejuvenate the soil, so I thought I would try the same with my spent compost!

OK as I was soon to find out, there was a bit of a learning curve for example, establishing a recipe on how to add the correct amount of humus and fertilisers to the spent compost, so I plumped for the John Innes method which is;
7 parts Loam
3 parts Peat
2 parts Coarse sand or Grit
To these I was required to add
2 parts, hoof and horn. (N)
2 parts superphosphate. (P)
1 part sulphate of potash. (K)
1 part ground limestone to control the pH**

To simplify this one can now purchase pre-formulated 'all in one' mixes which takes away the need to purchase Hoof & Horn, Superphosphate and Sulphate of Potash separately.

This was the point where I chose to differ slightly from the manner on how commercial mixes were made up.

I substituted Spent Compost for the Loam content, well-rotted Horse Manure for the Peat content and Perlite for the Sand/Grit. (I find sand makes the finished product too gritty!)

** In terms of establishing the pH I prefer to test the Loam, Manure and sand/grit after mixing then adjust the mixture with limestone as required.

I found with this recipe that the initial pH can be quite variable once the ingredients have been mixed together.

This is the area I had a problem with because I did not know how to calculate how much hydrated lime / limestone to add to my base mix to obtain a pH of somewhere around 6.0 to 7.5 and because of this I just used the 'Trial & Error' method.

For example, I have an analogue pH Tester where the probe gives me some guidance, although I am convinced it is reading around 1pH unit too high based on tests I have made with non-bleed pH indicator strips.

I am pleased to say my results are getting somewhat more consistent.

How I did this initially was to work with a 10litre bucket of 'Base Mix' which I tested with the pH tester and a pH indicator strip and recorded the pH result.

Next, I mixed in two 15ml spoonsful of lime to the 10 litres of 'Base Mix’ and tested it again with the pH tester and a pH strip and recorded the reading.

I carried out several tests in this manner where I added varying amounts of lime subject to the +/-pH readings I recorded from each batch.

Eventually I found a correlation in the results to show approximately how much lime I had to add to get to my target of 6.5.

To simplify this even more, I started making 40litre batches to reduce the number of tests I had to make.

I purposely do not add fertiliser at this stage as I prefer adding this when required, i.e., when I find I need a compost mix that is the equivalent of either Ji 1, 2, or 3 compost.

All I do is add the required amount of base fertiliser to a given quantity of my "Base Mix" and I have the desired compost I want.

In terms of cost, Ji 1 is costing me around = 3p/litre – Ji2= 3½p/litre – ji 4p/litre.

Hope I have not bored you all too much, but as I was making some notes for my records (my memory is not as good as it once was)I thought I would let you know what I am getting up to, now that I don't have an allotment

[Tee Gee]

Tried yesterday to add a few images, but they were too large, maybe his time

[woodypecks]

Enjoyed reading your thoughts and experiments on potting compost Tee Gee . It is becoming expensive .... not only that but it is so darned heavy to move about too !  Debbie 

[Tulipa]

And it's good to be re-using the compost, I am definitely keen to start something off this year, thanks for all your info Tee Gee 

[Beersmith]

An interesting contribution.

Would you be able to say a little more about the physical properties, like drainage, water retention, aeration and similar for your formula?  These are the aspects of  the modern peat free composts that are usually so much worse than the peat containing types.   

[Tee Gee]

Would you be able to say a little more about the

physical properties,

Essentially, the compost is like any peat based compost we have all used in the past, which is not surprising considering I save my spent compost in Dustbins & Dalek type containers. (see previous pictures)

The main difference is it is free of all the detritus that often came with multipurpose & potting composts due to me putting it through a riddle. I am expecting (hoping) that riddling will reduce in future, as I am only expecting to find remnants of roots from plants that grew in it prior to saving the spent compost.               

like drainage,

In terms of drainage again, this is similar to previous compost I purchased, although I have added around 5% Perlite (by volume) as I am working to John Innes formulae. Having said that, the main reason for using Perlite is I prefer it to coarse sand (River sand) plus it has some moisture holding attributes, and is not as heavy to lug around.

water retention & aeration ⁣

In my first trials, I used sieved multipurpose compost to add humus/fibre to the mix but this year I have added well rotted HORSE manure (as opposed to other types e.g. Cow, Pig and compost from a compost heap)That is not to say those that I listed would not work its just that I can source plenty of free horse muck.

This year is my first year with this recipe, so I will keep a close eye on how it performs.

For starters, I am using a mix of 3 parts cleaned up spent compost to one part horse manure, and as mentioned 5% Perlite.

At this stage I bag up the finished product and refer to it as my "Base Mix" and for all intents and purpose contains no nutrients/fertilisers other than perhaps some residual stuff from the previous year and what might be in the horse manure.

This means that if I want a 'Seed compost' I just use the base mix as it is.
Perhaps if I think I want the mix to be of a finer texture, I might put it through a finer sieve/riddle, time will tell.

This year I put this mix into an old Microwave oven I have with a view to sterilising it. At this stage, I haven't decided if I will use this process next year, for the simple reason I had a bit of a problem this year with my seedlings after germination in so far as the root system did not develop as I would have liked.

Having said that, I am keeping an open mind on this, as it might not have been as a result of the microwaving. I have a gut feeling that the compost was too wet and the new root systems were drowned.
To check on this next time, I have invested in a moisture meter to check the seed compost before and during the germination period.I am also using the meter to establish the moisture content of my 'container' grown plants prior to watering.
As I see it, this process will keep me informed of the compost's moisture content over the whole season.
If needs be, I will adjust the spent compost/manure and Perlite content to suit my findings.

Other recipes;

All I will do here is add the required amount of John Innes base fertiliser to the required amount of Base mix as needed to produce John Innes 1/2/3

Having said that, I have made a slight change to the fertiliser content (for economic reasons)

I had a look at the NPK of the John Innes Base fertiliser and Blood, Fish & Bone fertiliser, and they had very similar NPKs.

Not that I am particularly an 'organic' person but in the past I always used Fish, Blood & Bone (organic) if I was growing 'edible' stuff and I tended to use Growmore (non-organic) on non-edible stuff so what I have done is part way through my potting on procedures I used 100% John Innes Base Fertiliser, whereas later in the season I have used a mixture of 50-50 John Innes Base Fertiliser and Fish, Blood & Bone to see if I can detect any differences with the fertiliser content.

So that is as far as have gone with my experiments, so only time will tell.

pH

All the way through the exercise, I regularly check the pH of the mixes to ensure I am reading around 6.5.

According to the experts, the optimum pH for most garden plants and vegetables is 6.5 (slightly acid).

So I am constantly checking the mixture with my pH meter at its various stages of production.

Occasionally I get a bit more scientific and test with litmus strips.

Subject to my findings, I add lime as necessary.

Finally, I hope this reply answers some of the thoughts you may have in your minds.


 PS, I don't know if this will work, but I did a bit of a video at the beginning of last year. Things have moved on since then and are slightly different to what I have written in the text above. this is a link to it if it works

https://photos.onedrive.com/share/B213F524C3C600C0!307963?cid=B213F524C3C600C0&authkey=%21AkgWqHxVZHMqx44&ithint=video&e=4DSzfuc6AkG6Xi_frvqFzA&at=9

[Tulipa]

Thanks Tee Gee, I am really finding this interesting and had saved all my potting compost from last year so I can start experimenting. I am very late to gardening this year after a chest infection followed by Covid but getting some energy now I hope!